The present invention is directed to an optical communication network, and more particularly to a fiber optic network formed by interconnecting a number of passive transmissive stars, each star serving terminal devices in its local area.
Continuing improvements in the transmission quality of optical fibers, and in particular increased bandwidth and reduced attenuation rates, have made optical fiber communication networks an increasingly attractive alternative to networks which employ conductors as the transmission medium. In order to communicate optically, an electrical signal developed within a transmitting terminal device, such as, for example, a telephone, computer, or numerically controlled machine tool, is delivered to a transceiver within the terminal device. The transceiver uses the electrical signal to modulate light from a source such as LED or laser. The modulated light is then transmitted via an optical fiber to a transceiver within a receiving terminal device. The transceiver includes an optical detector, such as a photodiode, which reconverts the modulated optical signal into an electrical signal. Thus the transceivers within the terminal devices and the optical fibers connecting them effectively replace conductors which might otherwise have been used. Like conductors, optical fibers can transmit information in either analog or digital form.
A transmissive fiber optic star is a passive coupling device used to interconnect a number of terminal devices in a network. The physical structure of such a star is illustrated schematically in FIG. 1A, wherein four optical fibers have been fused at a tappered region 20 to provide a star 22 having light input ports 24, 26, 28, and 30 and light exit ports 32, 34, 36, and 38. Light entering star 22 through any of the input points 24-30 is equally distributed to all of the exit ports 32-38. For example, if light having an intensity of one unit were introduced into input port 24, light having an intensity of one quarter unit (neglecting minor losses) would be admitted through each of exit ports 32-38. Star 22 could be used to interconnect four terminal devices, each terminal device being separately connected via optical fibers to one of the input ports and one of the output ports.
Stars are not limited to four pairs of ports, as in the example of FIG. 1A. However the number of terminal devices that can be interconnected via a single star is generally under 80. This limitation is caused partly by difficulties in fabricating larger stars (i.e., stars with more than 80 pairs of ports). Another limiting consideration is that the optical power available at each light exit port is inversely proportional to the total number of exit ports. Thus the available transceiver sensitivity effectively imposes a limitation upon the star itself.
FIG. 1B schematically illustrates a fiber optic communication network employing a star 40. In this Figure, terminal device 42 in Local Area A is connected to a light input port and a light exit port of star 40 by fibers 44 and 46, respectively. Similarly, terminal devices 48 and 50 in Local Area B are connected to star 40 by fibers 52, 54, 56 and 58. Each terminal device has a transceiver, although only transceiver 60 in terminal device 42 is illustrated. Transceiver 60 includes an optical transmitter 62 which receives digital electrical signals from terminal device 42 and launches optical signals on fiber 44. Transceiver 60 also includes an optical receiver 64 which receives optical signals from fiber 46 and provides digital electrical signals to terminal device 42.
Sophisticated digital communication techniques have been applied to optical networks, and it will be apparent that transmitter 62 and receiver 64 may be correspondingly sophisticated. Their precise configurations would depend upon such matters as the nature of the digital coding scheme employed and the communication protocol. However, as a simple example, FIG. 1C illustrates a transceiver 66 having an optical transmitter consisting of driver amplifier 67 which receives a bit stream from the associated terminal device via conductors 70 and an LED 68 which emits a corresponding stream of ON or OFF light pulses to the end of fiber 72. The optical receiver in transceiver 66 includes an optical detector 74, such as a silicon photodiode, which receives pulses of light emitted from the end of fiber 76. The electrical signal from detector 74 is amplified by amplifier 78 and given sharp rising and falling edges by waveshaping circuit 80, such as a Schmidt trigger, which supplies digital data in electrical form to the associated terminal device.
FIG. 1B illustrates a primary problem which is encountered in single star fiber optic networks. If the terminal devices are widely dispersed, a large amount of fiber is required to run a separate pair of fibers from the star to each terminal device. This increases cabling complexity and network costs. For example if Local Area A represents a suite of offices in one building and Local Area B represents a suite of offices in a building a block away, an appreciable amount of fiber would be required to interconnect as few as 10 terminal devices in Local Area A and another 10 terminal devices in Local Area B. It will be apparent that, although the schematic symbol for star 40 suggests only 4 pairs of ports, which could be used to interconnect only four terminal devices, no such limitation is intended. As was mentioned above the capacity of the star is frequently significantly greater, and in practice star 40 would typically be used to interconnect more than the three terminal devices illustrated in FIG. 1B.
As is illustrated in FIG. 2A, one might attempt to reduce the amount of fiber required to interconnect a plurality of terminal devices 82 in Local Area A with a plurality of terminal devices 83 in Local Area B by using a star 84 positioned within Local Area A and a star 86 positioned within Local Area B, the stars 84 and 86 being interconnected by a single pair of fibers. A pulse of light emitted by one of the terminal devices in Local Area A, for example, would be distributed to the remaining terminal devices 82 by star 84. Star 84 would also distribute the pulse to star 86, which in turn would distribute the signal to the terminal devices 83 in Local Area B. However, if star 84 had N pairs of ports, only 1/N of the optical power would be delivered to star 86. If star 86 also had N pairs of ports, it will be apparent that the optical power provided to each of devices 83 would be only 1/N.sup.2. If there were three stars, the factor would be N.sup.3, etc. The repeated power attenuation would render the configuration of FIG. 2A unsatisfactory in a typical practical application.
One might attempt to avoid this power reduction by providing repeaters 88 in the optical fibers connecting stars 84 and 86, as is illustrated in FIG. 2B. In general a repeater includes an optical receiver portion and an optical transmitter portion. This general configuration is illustrated in FIG. 2C, wherein optical signals from fiber 90 are provided to optical receiver portion 92, the output of which is an electrical signal corresponding to the optical signal. This electrical signal is then re-converted to an optical signal by optical transmitter portion 94, the optical signal being emitted on fiber 95. As was the case of the optical transmitter and optical receiver within the transceiver of a terminal device, the particular configurations of optical receiver portion 92 and optical transmitter portion 94 of the repeater depend upon the nature of the communication network. However, in a network employing the transceivers 66 of FIG. 1C optical receiver portion 92 of a repeater might consist of a series connection of an optical detector, an amplifier, and a waveshaping circuit, with optical transmitter portion 94 being a driver amplifier and LED.
Returning to FIG. 2B, it will be apparent that the insertion of repeaters 88 between stars 84 and 86 in an attempt to overcome the power division problem discussed above would be unsatisfactory. A pulse of light emitted "upstream" by star 84 would be regenerated by repeater 88 before reaching star 86, which would return the pulse downstream to star 84. Since the repeaters 88 must have sufficient sensitivity to detect an input signal from any terminal device connected to one star and sufficient output power to transmit effectively to any terminal device connected to the other star, the closed loop configuration of FIG. 2B would result in instability. Essentially, each repeater 88 endlessly repeats the output of the other repeater 88.
One might attempt to avoid this problem by electrically connecting the repeaters 88 so that they cannot both be operative simultaneously. This expedient, however, would not entirely avoid the problem. Assume, for example, that a pulse stream is emitted from star 84 to star 86 and that the bottom repeater 88 deactivates the top repeater 88 as the bit stream passes. Due to propagation delays, however, the top repeater 88 would become operative before completion of the signal reflection by star 86. In order to ensure reliable operation it would be necessary to deactivate one repeater 88 for an additional period following the period in which the bottom repeater 88 is operative. The additional deactivation period required would equal the time required for optical signals to propagate from the bottom repeater 88 through star 86 to the top repeater 88. The increased delay would be significant if long fiber lengths are involved. Further, for signals propagating through the top repeater 88, a different period of deactivation might be required for the bottom repeater 88, since this would depend on the signal propagation time from repeater to repeater through star 84, which might be at a different distance from the repeaters than is star 86.